1. Field of the Invention
The present invention generally relates to multi-band radio antennas and, more particularly, to direction finding antennas which are compact and suitable for use on vehicles under adverse conditions.
2. Description of the Prior Art
Wireless communications are currently used for many applications including data transmission, resource tracking, and emergency or safety services. Many of these applications involve determining the location or the direction of a transmitter of radio frequency signals which may be of any frequency over a broad spectrum. However, antenna efficiency and directional selectivity are dependent on the relationship of the frequency of the radio carrier signal and the dimensions of the antenna elements and the distances between them and, in general, efficiency of a given antenna system will be acceptable only over a relatively limited band of frequencies.
Therefore, multiple frequency band antenna subsystems are typically used to cover wide frequency range applications. However, the use of multiple antenna subsystems require significant physical separation between them to ensure adequate direction finding performance that may be degraded by interactions between such antenna subsystems. Performance can also be degraded by adverse mast resonance effects caused by common mode coupling of antenna subsystem elements to the mast. This mast resonance effect, if not controlled could cause dramatic phase change with relatively small frequency change that results in unreliable calibration and/or ambiguity of direction/azimuth in regard to the transmitted signal. It is well-recognized that this required separation of antenna subsystems imposes size restrictions for low frequency operation and vice-versa. Large size also often implies significant weight of the overall antenna system. Since many direction finding applications involve vehicles which must carry the antenna system, practical limitations on both size and weight may be imposed.
Further, land vehicles used in such applications are often operated in off-road environments where large antenna system size also implies an increased susceptibility to damage (e.g. from tree limbs and the like). Large, vehicle mounted antennas also generally require some arrangement for reducing shock and vibration due to terrain effects on the vehicle. Additionally, the antenna system and its components will exhibit degradation in performance due to temperature variation, particularly extreme temperature rise due to solar loading.
Even relatively compact antenna designs which are known may be of substantial size. For example, so-called bowtie antennas which are broad band radiators may be used in a cylindrical array but the bowtie element width must be large for adequate radiation efficiency at lower frequency ranges which adversely results in an electrically large diameter of the cylindrical array at a higher frequency range. This diameter of the cylindrical array limits the maximum frequency bandwidth operation of this antenna subsystem. In other words, this type of array design is not operated in an optimal frequency range and direction finding accuracy would be degraded.
In summary, particularly for direction finding applications, antenna system design must seek to answer numerous demands within a large and restrictive group of environmental constraints which are related in numerous complex ways and requiring numerous trade-offs between performance and physical configuration. Because of these trade-offs, no known design has been compact in size while having improved direction finding performance over a wide frequency band covering the VHF, SHF and UHF ranges, while being resistant to damage and providing good thermal and mechanical performance including reduced solar loading, improved heat dissipation from active elements and vibration damping and which can be readily and unambiguously calibrated.
In this regard, many details of known antenna systems are relatively critical to individual antennas of any given design. A change in routing of only a minor length of wire or a slight shift in relative location of elements and/or components of any given antenna system design may significantly alter antenna performance, particularly phase shift with frequency. Therefore, for direction finding applications, antenna systems must be individually calibrated. Some of these details, such as wire routing are particularly subject to change during repairs and it is generally the case with known antenna designs that the antenna must be recalibrated after any repair. Such recalibration requires specialized equipment and cannot generally be accomplished in the field.
It is therefore an object of the present invention to provide a direction finding antenna which is compact in size while having improved direction finding performance over a wide frequency band covering the VHF, SHF and UHF ranges (from below 25 MHz to above 2000 MHz), while being resistant to damage and providing good thermal and mechanical performance including reduced solar loading, improved heat dissipation from active elements and vibration damping.
It is another object of the invention to provide a compact, broad band direction finding antenna system suitable for use on a land vehicle and which exhibits common mode coupling to the mast which is shifted out of the antenna subsystem frequency band.
It is a further object of the present invention to provide a wide band antenna system which has improved characteristics for being transported, improved interoperability and which can be manufactured more economically and serviced in the field without recalibration.
In order to accomplish these and other objects of the invention, an antenna system is provided including a mast, a plurality of antenna arrays colocated on the mast but separated from each other antenna array by a distance along the mast, and a loaded inductor or the like for shifting a frequency of mast resonance out of the frequency bands of said plurality of antenna arrays.
Shaping of the VHF dipole array reduces scattering to other arrays while providing mechanical advantages including damping of vibration and protection for another array. The UHF array includes angled bowtie elements to improve frequency and phase response. other perfecting features of the preferred embodiment of the invention include fixtures for locating all removable electrical parts including wiring and components enclosed in a heat dissipating, finned RF electronics housing so that servicing and repair can be performed without recalibration and parts are made substantially interoperable. Asymmetrical keyed elements and connection arrangements assure alignment of antenna elements, antenna arrays and the entire antenna system.